1. Field of the Invention
The present invention relates to a semiconductor memory device that is provided with antifuses that are used in remedying memory cells in which defects have been found, and to a test method for this semiconductor memory device.
2. Description of the Related Art
In semiconductor memory devices in recent years, the miniaturization of memory cells and peripheral circuits that has accompanied the great increase in memory capacity has brought with it an increase in the incidence of defective parts that are unintentionally incorporated. Semiconductor memory device are therefore provided with both normal memory cells, which are memory cells for normal use, and redundant memory cells, which substitute when a defect occurs in a normal memory cell; and the defect remedy technology for replacing normal memory cells in which defects have been detected (hereinbelow referred to as “defective memory cells”) with redundant memory cells to improve product yield has become a crucial technology.
In order to replace a defective memory cell with a redundant memory cell, the address of the defective memory cell (hereinbelow referred to as “fail address”) must be stored. Nonvolatile memory elements for storing fail addresses include fuses in which a conductor is fused and insulated by a laser or electric current, and antifuses in which an overvoltage is applied to an insulator to break down the insulator to cause conduction.
In fuses that are typically used as in semiconductor memory devices of the prior art, a laser light is used when testing the memory chip to fuse conductors and thus store fail addresses, and as memory elements, these fuses have relatively stable characteristics. However, a method in which a laser is used to melt a fuse and store fail addresses suffers from the problem that, once a memory chip has been sealed within a package (assembled), defects that are detected can no longer be remedied, and this method is therefore unable to adequately improve product yield. In recent years, however, antifuses are coming into use that are capable of remedying defective memory cells even after assembly.
As a typical method for storing fail addresses in an antifuse, fail addresses are first stored to an internal register (hereinbelow referred to as an “address register”) that is provided in the semiconductor memory device, following which antifuses are short-circuited (hereinbelow referred to as “programmed”) in accordance with fail addresses that have been stored in the address register. Methods for storing fail addresses in the address register include a method in which, each time a defect is detected during a test of memory cells, the fail address is written to the address register inside the semiconductor memory device; and a method in which a known memory test is used to detect fail addresses in advance, and the detected fail addresses then written to the address register from the outside.
As a typical method for programming antifuses, the semiconductor memory device is switched to the programming mode, and voltage pulses for breaking down insulation are supplied to the antifuses.
An example of a configuration for detecting the state of antifuses following programming for improving the efficiency of the programming step is described in JP-A-2000-90689.
Of the above-described methods for storing fail addresses in address registers, the method in which fail addresses are written to the address registers inside the semiconductor memory device each time a defect is detected lacks a means for checking whether the number of detected fail addresses is greater than the number of antifuses that are provided in the semiconductor memory device, and as a result, the inability to remedy defective memory cells is not discovered until the test step that follows completion of programming of antifuses. As a result, there is the problem that pointless tests are applied to semiconductor memory devices in which defective memory cells cannot be remedied.
In the method in which fail addresses that have been detected in advance are written to the address register from the outside, moreover, as previously described, a known memory tester is used to detect fail addresses in advance, the detected fail addresses are written to the address register from the outside, and the antifuses then programmed, following which a test step is performed to verify whether defective memory cells have been remedied or not. As a result, there is the problem of inconvenience in the remedy routine for remedying defective memory cells.
In addition, when a defect remedy process using antifuses is carried out a plurality of times, such as when a defect remedy process is carried out for a memory chip by itself, and then for the memory chip after it has been sealed in a package, the remaining antifuses that have not been programmed in the previous defect remedy processes must be verified. However, the problem arises that when the remaining antifuses cannot be verified, the same programming process is repeated in the second and succeeding defect remedy processes for already programmed antifuses, resulting in an unnecessarily lengthy defect remedy process.